Multilink frame relay connections provide several advantages over the traditional asynchronous transfer mode (ATM) of communication. While ATM can be used to send data over multiple links, by using inverse multiplexing over ATM (IMA), usually the data cells sent must all be of the same size. Additionally, ATM usually requires the links to have the same speed of transfer. Multilink has no such limitations to size and speed.
Large frames are usually fragmented to prevent clogging of the bundle link. A timer is activated when fragments arrive out of sequence, with interior fragments missing. If the timer finishes before a missing fragment reaches the destination, the entire frame is lost.
Previously, multilink connections usually used either a round robin or a credit method to determine assign data frames or data frame fragments. Exemplary results of both the round robin method and the credit method are illustrated in FIGS. 1a and 1b. In this example, a transmitter 100 breaks the data frame into six fragments, labeled 101, 102, 103, 104, 105, and 106. The multilink data frame relay connection has three bundle links, each of separate speed ratings. Link 110 has a rating of 1 ds0, link 120 is twice as fast with a rating of 2ds0, and link 130 is three times as fast with a rating of 3ds0. A receiver 140 receives the frame fragments and reassembles the frame. In one embodiment, the transmitter and the receiver are transceivers.
In the round robin method, illustrated in FIG. 1a, a transmitter 100 transmits data frame fragments chronologically into each bundle link, repeating when all links have been used. This method results in fragments 101 and 104 being transmitted in link 110, fragments 102 and 105 being transmitted in link 120, and fragments 103 and 106 being transmitted in link 130. Because of the differentiated speed between links, the order of arrival is then 103, 102, 106, 101, 104, and 105, with 101, 104, and 105 all arriving simultaneously. The fragments arriving in this order would activate the timer, and possibly the whole frame would be lost.
In the credit method, illustrated in FIG. 1b, bundle links are weighted according to link speed. Frames and frame fragments are sent on the fastest link, link 130 in this example, until a threshold is exceeded, at which point the frames and frame fragments are sent on the next fastest link, link 120. This method results in frame fragments 101, 102, and 103 being transmitted on link 130; frame fragments 104 and 105 being transmitted on link 120; and frame fragment 106 being transmitted on link 110. The order of arrival becomes 101, 104, 102, 106, 105, and 103, with 106, 105, and 103 arriving simultaneously. This order also activates the timer and endangers capture of the entire frame.
The reason for this is that, while link 130 transfers data much faster than either link 120 or link 110, as more frame and frame fragments are transmitted through a link, the link is slowed down. The phenomenon is similar to lanes in a freeway. The carpool lane may allow a car to travel faster, but as more cars load into the lane, the speed slows down.